Acousto-optic devices have long been known in which a LiNbO.sub.3 crystal is electrically driven by an interdigitated electrode structure formed on its surface. The resultant surface crystal vibrations (surface acoustic waves or SAW) interact with light traversing the LiNbO.sub.3 crystal. Thereby the light can be electrically controlled by the following method. The LiNbO.sub.3 substrate is intentionally birefringent and oriented such that the horizontal (TE) mode and the vertical (TM) mode propagate at different speeds. These polarization states fall in and out of phase over such a short distance, referred to as the beat length, that energy cannot be transferred between them. However, the electrical driven double-comb structure imposes a periodic, alternating stress in the piezo-electric, photo-elastic substrate. If the period of this electrically applied stress is made synchronous with the beat length, energy is efficiently transferred between the TE and TM modes. The interaction depends on the electrical frequency matching the light frequency with physical parameters of the LiNbO.sub.3 being included as multiplicative factors. Thereby, the optical frequency can be electrically selected and that selected light component has its polarization converted between the TE and TM modes. By judicious placement of polarizers there results an electrically selectable light filter. Although bulk LiNbO.sub.3 has received the majority of attention, similar effects have been reported in bulk TeO.sub.2 and CaMoO.sub.4. Further, the piezo-electric effect is used to convert the applied voltage to a crystalline stress. Therefore, acousto-optic devices may be fabricated in a birefringent, photo-elastic, but non-piezo-electric substrate if a piezo-electric buffer layer is interposed between the substrate and the electrodes.
Most of the early LiNbO.sub.3 acousto-optic devices, such as the tunable acousto-optical filter originally disclosed by S. E. Harris et al. in a technical article entitled "Acousto-optic tunable filter" appearing in Journal of the Optical Society of America, vol. 59, 1969 at pp. 744-747, were bulk devices, relying on both bulk acoustic waves and bulk optical waves. Progress was then made in using surface acoustic waves (SAW) for which the acoustic waves were pinned to the LiNbO.sub.3 surface. However, bulk acoustic waves have a very large cross-section (typically with a beam diameter of greater than 1 mm), and to obtain significant acousto-optic interaction without an attempt to guide both the light and the acoustic waves adjacent the surface, acousto-optic devices were heavy power consumers. With these bulk optic devices, usually all other optical and electrical components were external to the LiNbO.sub.3 crystal. With the advent of integrated option and opto-electronic integrated circuits, there has arisen a desire to integrate LiNbO.sub.3 devices into these circuits and furthermore to reduce the power consumption. That is the light should be guided near the surface and the acoustic waves in the LiNbO.sub.3 should be localized. For effective integration with an optical integrated circuit, the acousto-optic device should guide light in a single-mode waveguide. The progress to acousto-optic devices with surface acoustic waves and planar optics has been slow and commercial devices have tended to rely on bulk waves.
An early example of integrated acousto-optic devices was disclosed by Y. Omachi et al. in a technical article entitled "Acousto-optic TE-TM mode conversion using collinear acousto-optic interaction" appearing in IEEE Journal, volume QE-13, 1977 at pp. 43-46. Omachi et al. followed the usual prior art practice of using an y-cut LiNbO.sub.3 crystalline substrate. LiNbO.sub.3 forms a trigonal crystal at the temperatures at which it is used for acousto-optic devices. R. S. Weis et al. have described in detail the crystal structure of LiNbO.sub.3 and its photo-elastic end piezo-electric effects in a technical article entitled "Lithium Niobate: Summary of Physical Properties and Crystal Structure" appearing in Applied Physics A, vol. 37, 1985, at pp. 191-203. The disclose the standard x, y, and z principle axes of LiNbO.sub.3. A LiNbO.sub.3 crystal having a principle face lying in a plane perpendicular to the x-axis is x-cut LiNbO.sub.3.
The advantage of a y-cut LiNbO.sub.3 substrate is that the optical and acoustic waves propagate collinearly along the x-direction on its surface with the acoustic power flow being collinear with the acoustic wavefront propagation vector. However, high quality optical waveguide are difficult to fabricate in y-cut LiNbO.sub.3 and x-cut LiNbO.sub.3 is the subject of a more mature fabrication technology. G. D. Boyd et al. have similarly concluded in a technical article entitled "Tunable Acoustooptic Reflection Filters in LiNbO.sub.3 Without a Doppler Shift", appearing in Journal of Lightwave Technology, vol. 7, 1989 at pp. 625-631 that x-cut LiNbO.sub.3 is preferred. On the other hand, the x-cut has been avoided because a y-directed acoustic wave on an x-cut walks off and rectilinearly propagates at an appreciable angle from the y-propagating optical wave. Such walk-off limits the acousto-optic interaction length. Boyd et al. proposed solving the 4.degree. walk-off problem by orienting the acoustic transducer at approximately 4.degree.. As will be shown later, this solution is only approximate at small walk-off angles and becomes inadequate at larger walk-off angles.
As described above, one of the primary uses of acousto-optical devices is to provide a tunable optical filter. The transfer function for an acousto-optic filter at a wavelength displacement .DELTA..lambda. from its peak wavelength .lambda. is given by ##EQU1## In this equation, EQU x=((1+.phi..sup.2)).sup.1/2
where ##EQU2## is the detuning parameter for which ##EQU3## is the TE-TM polarization beat length, L is the interaction length and .DELTA.n is the effective waveguide birefringence. The TE-TM mode coupling coefficient k is given by ##EQU4## where P is the acoustic power density and P.sub.o is that acoustic power density required for complete TE-TM mode conversion. It is seen from the above equations that, by making the interaction length L relatively long, the filter can be made very narrow band. In the infrared optical communications band between 1200 nm and 1600 nm, 1 nm FWHM bandwidths have been observed for the central pass band. However, the above equation also show that, regardless of the bandwidth, the relative size of the sidebands remains the same. The first sideband is reduced by only 9 dB from the main pass band.
One of the desired applications of acousto-optic filters in the telecommunications industry is for wavelength-division multiplexing (WDM) fiber optic systems in which different channels at a different wavelengths are transported on a single fiber. The acousto-optic filter or other acousto-optic device then selects one of the channels for filtering or other type of coupling to adjacent media. Such systems have been proposed in commonly assigned U.S. patent applications, Ser. Nos. 07/292,021, filed Dec. 30, 1988 by Cheung P. T. 4,906,064 and 07/324,184, filed Mar. 16, 1989 by Cheung et al.
When acousto-optic devices are applied to more demanding optical applications, an inherent limitation of the prior art single-stage acousto-optic devices arises. The filtering of the optical beam is performed through an acousto-optic interaction that produces a polarization conversion. The mode conversion is achieved by means of a momentum transfer (and associated energy transfer) from the acoustic wave to the optical wave. The energy transfer results in an optical frequency shift .DELTA.f from the input value f.sub.o equal in magnitude to the acoustic frequency f.sub.a. This shift is sometimes referred to as a Doppler shift. The sign of the shift .DELTA.f depends on the input polarization and whether the directions of light and sound propagation are collinear or contralinear. A typical value of f.sub.a is 350 MHz or 10.sup.-3 nm. This effect is fully explained by Dixon in a technical article entitled "Acoustic Diffraction of Light in Anisotropic Media" appearing in IEEE Journal of Quantum Electronics, volume QE-3, 1967 at pp. 85-93.
The idea of using acousto-optical tunable filters as a tuning element for external cavity lasers has been initiated by Taylor et al. in a technical paper entitled "Electronic tuning of a dye laser using the acousto-optic filter", appearing in Applied Physics Letters, vol. 19, 1971 at pp. 269-271. In that paper, they used an CaMoO.sub.4 collinear acousto-optic tunable filter as the tuning element and used it to tune a dye laser at 0.54-0.63 .mu.m. However, the frequency shift of the optical beam during the acousto-optical interaction, as described above, poses a problem. After the optical beam is reflected back from the external cavity into the laser gain medium, the optical frequency of the beam is shifted (either upshift or downshift, depending on the configuration of the filter) by two times the acoustic frequency f.sub.a. Such a frequency shift causes undesirable continuous chirping mode changes, and single frequency operation is impossible. The power output is also unsteady because of the chirping nature.
G. A. Coquin et al. have proposed a solution for the frequency shift in a technical article entitled "Electronically tunable external-cavity semiconductor laser" appearing in Electronics Letters, vol. 24, 1988 at pp. 599-600. They proposed cascading within the laser cavity two tunable acousto-optic filters with equal but opposite frequency shifts. However, Coquin et al. used to separate acousto-optic devices so that the system is bulky. More importantly, the separately fabricated acousto-optic converters must be very precisely matched if they are to mode-convert exactly the same ranges of optical wavelengths and the same time produce equal and opposite Doppler shifts.
The above described dependence of frequency shift on input polarization and the general observation that conventional acousto-optic devices rely upon the state of polarization of the optical input point out a further difficulty with acousto-optic devices. They are polarization dependent.
With the widespread deployment of standard non-polarization-preserving single-mode fibers int he rapidly expanding optical communications networks, the requirement of polarization insensitivity in remote and receiver optical processing components becomes necessary. Without such polarization insensitivity, the insertion loss and efficiency of devices so placed become uncontrollably degraded, time dependent and wavelength dependent.
In these optical telecommunication networks, the capacity of fibers can be greatly increased by wavelength division multiplexing. The use of acousto-optic filters has been proposed. However, because of the fixed and relatively large size of the sidebands for acousto-optic filters, the frequencies of the channels must be carefully selected so that neighboring channels fall in the nulls of the transfer function. Further, there has remained the unsolved problem of switching separate channels into and out of a fiber or other optical waveguide already carrying signals at different frequencies. Particularly constraining has been absence of a practical wavelength-selective tap or wavelength-selective routing component capable of switching more than just on single selected channel at a time. The solutions available up to the present have been considered infeasible. If multiple WDM channels are to be simultaneously switched, multiple RF driver frequencies are simultaneously applied to the acousto-optic device. In order to remain within power dissipation limits, such a multiple-channel switcher must be more efficient than that commonly available in the prior art.